Field of the Invention
This invention relates to a new type of suction leg which is used as foundation of offshore caissons or platforms.
Background of the Invention
Currently, the most common foundations of an offshore fixed structure have following four types.
1. Self-elevating platform leg bearing loads after inserted into seabed, which is widely used and includes steel tube friction leg (rarely used now) and boot leg, and the boot leg rely on its pile shoe to bear loads. The biggest advantage of the self-elevating platform is that it does not need any offshore construction facility such as floating crane during offshore installation and the relocation process besides tugs, that is to say the self-elevating platform can achieve self-installation and self-relocation. However, the disadvantage is the depth of the leg inserted into mud is shallow, usually less than 20 meters, which is due to a large resistance of each pile shoe into mud, and a limited volume of the water ballast tank on the platform deck to offer a smaller weight required for the leg inserted into mud. Therefore, the bearing capacity of the self-elevating platform leg, especially the horizontal bearing capacity is limited. When the self-elevating platform serves as a production platform and is located at a same position without moving and relocation for several or a dozen years, it must be designed as per decades or one hundred years environmental conditions, which is a big challenge to ensure the leg's bearing capacity.
2. Open long pile, which is a two-end-opening steel or reinforced concrete long pipe, so called as open pile. The open long pile, such as a steel pile of a jacket platform, inserted into a pile sleeve of the offshore structure is driven into a seabed by a pile hammer and then fixed to the pile sleeve. Due to the limited capability of the pile hammer, the diameter of the open long pile is usually not more than 2.5 meters, and the length in mud is usually not more than 90 meters. The open long pile relies on the lateral friction force between the pile and the soil and the tip resistance after soil-consolidation to bear the vertical load, so the open long pile is usually called as friction pile. The main disadvantage is piling must use the pile hammer which needs a supporting offshore floating crane, and the open long pile will be difficult to pull out and reuse.
3. Suction pile, which is a bottom-opening and top-closed cylinder usually made of steel structure. During the suction or water injection processes by the special pump(s) installed on the top head of the suction pile, the downward penetration force or the upward uplifting force is produced by the pressure difference between inside and outside the top head, and the penetration force or the uplifting force would press or pull the suction pile into or out from the seabed. During penetration, the said pressure difference is determined by the water depth and difficult to increase so much, meanwhile a large external pressure may cause a buckling problem to the cylinder and its top head. Therefore, the diameter and penetrative depth of the suction pile have to be restricted, the diameter is usually 8 to 10 meters, the penetrative depth is generally not more than 12 meters, and the corresponding penetration force cannot be so large. Suction pile belongs to shallow foundation, which relies on the tip resistance, lateral earth pressure, lateral friction force and internal and external differential pressure to bear loads. The advantage of the suction pile is it can be pulled out and reused, that is to say it has recoverability. Two shortcomings of suction pile are as follows: firstly it need special pump and corresponding offshore operations supporting facilities, secondly it has high requirements on the shallow layers' bearing capacity of the seabed.
4. Gravity type foundation, which is widely used in offshore concrete gravity platforms, for example, a fixed offshore structures can stay on a seabed relied on its massive gravity. At present, offshore caissons used in ports, bridges and artificial islands also usually use gravity type foundations, supplemented by anti-sliding piles. The disadvantage of the gravity foundation is it has high requirements on the bearing capacity of the seabed and the structure is difficult to remove.
Fixed platforms for oil and gas field development in shallow waters mainly include jacket platform and gravity platform, mobile self-elevating platform, their basic structure and advantages/disadvantages are well-known and do not need to repeat. Usually, only the gravity platform has liquid storage function, and the mobile platform and the jacket platform, without storage, need to be compatible with FPSO(s) (floating production storage and offloading vessel(s)) to meet the requirement of oil drilling, oil production, storage and exportation. All of said existing facilities have high EPC investments, operating and decommissioning costs.
PCT/CN2013/070808 disclosed a type of sealing steel pile comprising a tubular pipe and a top head connected tightly to form a cylindrical integral structure with a sealing top and an opening bottom, and a release valve, an air intake valve and a water intake valve installed on the top head. As the foundation of offshore facilities, the sealing steel pile could be pressed into seabed by the weight of the facilities with ballast water, or/and by pump suctioning; and also, it could be pulled out from the seabed by the buoyancy of the facilities de-ballasted water, or/and by the method to inject water or gas into the sealing pile. PCT/CN2013/070808 gave detailed descriptions about the sealing pile pressed into or pulled out from seabed, as well as the offshore installation of the facility with sealing steel piles, and also disclosed limited information about the connection structure between the sealing steel piles and the offshore facility.